1. Field of the Invention
This invention relates generally to distance measurement devices, and in particular to distance measurement devices which may be retrofit onto vehicles.
2. Description of the Prior Art
Farmers in the United States of America have become increasingly dependent upon our federal government farm policy, as implemented by the United States Department of Agriculture. For example, the Agricultural Stabilization and Conservation Service of the Department of Agriculture provides cash subsidies to farmers in amounts dependent upon the number of acres in a farm system that are cultivated as compared to the number of acres that are fallow. It is the farmer's obligation to accurately measure the cultivated acreage. Such figures are submitted to the Department of Agriculture, which spot-checks some of the acreage measurements. Under current regulations, misstatements in the cultivated acreage cannot be corrected and will result in diminishment or loss of cash subsidies, without regard to whether such misstatements were deliberate falsehoods, or honest miscalculations.
The current Agricultural Stabilization and Conservation Service regulations require acreage measurements to be within five percent of the "true" cultivated acreage, as determined by the Department of Agriculture in spot-checks. This five percent tolerance standard cannot be met with confidence by standard vehicle odometers. Consequently, farmers rely upon measurement wheels which are handheld measurement devices. In operation, measurement wheels are dangled from a vehicle window by either the driver or a passenger and are pressed into contact with the earth's surface as the vehicle is advanced over the acreage that is to be measured. Measurement wheels present a variety of significant disadvantages but provide distance measurements that are far more accurate than vehicle odometers. The physical discomfort of the operator is a primary disadvantage when large tracts of land are being measured. In addition, the wheel is subject to inaccuracy, if not held in constant contact with the earth's surface. For example, when the wheel bounces, inaccuracy is introduced into the measurement. Moreover, measurement wheels can collide with the vehicle during operation, causing damage to the vehicle paint and body, not to mention the damage that can occur to the wheel. Finally, measurement wheels are too large to be conveniently stored in the vehicle cab, and therefore may not be readily available when measurements are desired.
Alternately, accurate acreage measurements may be made through aerial surveys of the farm land. However, aerial surveys are frequently expensive, and require the farmer to rely upon a surveyor to provide accurate measurements. Agricultural subsidies are such an important element in modern farm economics that some farmers will make acreage measurements using both the measurement wheel and aerial surveys, in order to ensure that important subsidies are not lost through mismeasurement.
A number of prior art distance measuring devices exist, including Kinney et al., U.S. Pat. No. 4,532,710, for Distance Measuring Device For Golf Carts and the Like; Zeaman et al., U.S. Pat. No. 4,680,454, for Odometer For Golf Cart; and Fleischer, U.S. Pat. No. 4,697,278, for Electronic Hub Odometer. In Kinney et al., a permanent magnet is mounted on a vehicle wheel, and a magnetic sensor is fixedly secured adjacent to the wheel for counting the passage of the permanent magnet as the wheel rotates. Likewise, in Zeaman et al., an odometer for a golf cart is provided which is attached to the wheel of the cart and counts revolutions of the wheel through use of a magnetic head that rotates past a stationary Hall Effect sensor. In Fleischer, a hub odometer is disclosed which employs a permanent magnet and a magnetic read switch for recording the number of revolutions of a vehicle wheel to which the odometer is mounted.
These prior art systems are unacceptable for vehicles which either travel over rough terrain or at high velocities, due to the requirement of a permanent magnet secured to the vehicle wheel. In rough terrain, of the kind encountered in cross-country driving, a permanent magnet affixed to the rotating wheel may become dislodged, causing inaccurate measurements. Furthermore, the coupling of a permanent magnet to the rotating wheel will upset the tire balance, result in premature and uneven tire wear, decrease the comfort of the ride particularly at high speeds, and diminish the safety of travel. In addition, the requirement of permanent magnets mounted to the vehicle wheel increase the complexity of retrofitting a measurement system onto a vehicle. Hub odometers are likewise subject to damage in rough terrain, and are further subject to water damage, since electronic components are disposed at wheel level. Likewise, hub odometers may upset the balance and performance of vehicles onto which they are mounted.